Fourteen percent of known U.S. natural gas reserves contain more than 4% nitrogen. The bulk of these reserves cannot be exploited because no economical technology for removing the nitrogen exists.
Cryogenic distillation is the only process being used to date on any scale to remove nitrogen from methane in natural or associated gas. Twelve such plants are believed to be in operation in the U.S., for example in enhanced oil recovery, where nitrogen is used to pressurize the formation and tends to build up in the associated gases removed with the oil. The gas streams that have been treated by cryogenic separation contain relatively large amounts of nitrogen, such as more than 10 vol %. Cryogenic plants can be cost effective in these applications because all the separated products have value. The propane, butane and heavier hydrocarbons can be recovered as natural gas liquids (NGL), the methane/ethane stream can be delivered to the gas pipeline and the nitrogen can be reinjected into the formation.
Cryogenic plants are not used more widely because they are expensive and complicated. A particular complication is the need for significant pretreatment to remove water vapor, carbon dioxide, and C.sub.3+ hydrocarbons and aromatics to avoid freezing of these components in the cryogenic section of the plant, which typically operates at temperatures down to -150.degree. C. The degree of pretreatment is far more elaborate and the demands placed upon it are far more stringent than would be required to render the gas acceptable in the pipeline grid absent the excess nitrogen content. For example, pipeline specification for water vapor is generally about 120 ppm; to be fit to enter a cryogenic plant, the gas must contain no more than 1-2 ppm of water vapor at most. Similarly, 2% carbon dioxide content may pass muster in the pipeline, whereas carbon dioxide may be present only at the level of 100 ppm or less for cryogenic separation.
Other processes that have been considered for performing this separation include pressure swing adsorption and lean oil absorption; none is believed to be in regular industrial use.
Gas separation by means of membranes is known. For example, numerous patents describe membranes and processes for separating oxygen or nitrogen from air, hydrogen from various gas streams and carbon dioxide from natural gas. Such processes are in industrial use, using glassy membranes. Rubbery membranes are used to separate organic components from air or other gas mixtures, such as in resource recovery and pollution control.
It is also known to combine membrane separation with cryogenic distillation. For example, the following U.S. patents show such processes for the separation of carbon dioxide from methane: 4,529,411; 4,511,382; 4,639,257; 4,599,096; 4,793,841; 4,602,477; 4,681,612; 4,936,887 and 5,414,190. U.S. Pat. No. 4,374,657 shows a combination of cryogenic distillation and membrane separation for separating ethane from carbon dioxide. U.S. Pat. No. 4,654,063 shows cryogenic separation followed by membrane separation for separating hydrogen from other gases. U.S. Pat. No. 4,595,405 shows a similar arrangement for separation of nitrogen and oxygen from air. U.S. Pat. Nos. 4,687,498 and 4,689,062 show process designs combining membrane separation and cryogenic distillation for recovery of argon from ammonia plant purge gas mixtures.
A report by SRI to the U.S. Department of Energy ("Energy Minimization of Separation Processes using Conventional Membrane/Hybrid Systems", D. E. Gottschlich et al., Final Report under Contract number DE 91-004710, 1990) suggests that separation of nitrogen from methane might be achieved by a hybrid membrane/pressure swing adsorption system. The report shows and considers several designs, assuming that a hypothetical nitrogen-selective membrane, with a selectivity for nitrogen over methane of 5 and a transmembrane methane flux of 1.times.10.sup.-6 cm.sup.3 (STP)/cm.sup.2 .multidot.s.multidot.cmHg, were to become available, which to date it has not.
In fact, there are several difficulties associated with separating nitrogen from methane by means of membranes, the main one being the absence of membranes with a useful selectivity. Both glassy and rubbery membranes have poor selectivities for nitrogen over methane or methane over nitrogen. Table 1 lists some representative values.
TABLE 1 ______________________________________ Permeability Selectivity (Barrer) (-) N.sub.2 CH.sub.4 N.sub.2 /CH.sub.4 CH.sub.4 /N.sub.2 Ref. ______________________________________ Polyimide (6FDA-mp'ODA) 0.26 0.13 2.1 0.5 1 Polyimide (6FDA-BAHF) 3.10 1.34 2.3 0.4 1 Polyimide (6FDA-IPDA) 1.34 0.70 1.9 0.5 2 Polyimide (6FDA-MDA) 0.20 0.10 2.0 0.5 3 Cellulose acetate 0.35 0.43 0.8 1.2 4 Polycarbonate 0.37 0.45 0.8 1.2 4 Polysulfone 0.14 0.23 0.6 1.7 4 Poly(dimethylsiloxane- 103 335 0.3 3.3 4 dimethylstyrene) Poly(dimethylsiloxane) 230 760 0.3 3.3 4 Poly(siloctylene-siloxane) 91 360 0.25 4.0 5 Poly(p-silphenylene-siloxane) 3 12 0.25 4.0 5 Polyamide-polyether copolymer 4.8 20 0.24 4.2 4 ______________________________________ 1 Barrer = 10.sup.-10 cm.sup.3 (STP) .multidot. cm/cm.sup.2 .multidot.s .multidot. cmHg References for table: 1. K. Tanaka, H. Kita, M. Okano, and K. Okamoto, "Permeability and Permselectivity of Gases in Fluorinated and Nonfluorinated Polyimides," Polymer 33, 585 (1992). 2. M. R. Coleman and W. J. Koros, "Isomeric Polyimides Based on Fluorinated Dianhydrides and Diamines for Gas Separation Applications," J Memb. Sci. 50, 285 (1990). 3. T. H. Kim, W. J. Koros, C. R. Husk, and K. C. O'Brien, "Relationship Between Gas Separation Properties and Chemical Structures in a Series of Aromatic Polyimides," J. Memb. Sci. 37, 45 (1988). 4. J. G. Wijmans, "Membrane Processes and Apparatus for Removing Vapors from Gas Streams," U.S. Pat. No. 5,071,451 (December 1991). 5. S. A. Stern, V. M. Shah, and B. J. Hardy, "Structure Permeability Relationships in Silicone Polymers," J. Polymer Sci: Polymer Physics Ed. 25, 1263, (1987).
These separation properties are not good enough to make membrane separation practical for this gas pair. With a nitrogen-selective membrane, we have calculated that a nitrogen/methane selectivity of about 15 is needed for a practical process that achieves adequate nitrogen removal and at the same time that avoids losing excessive amounts of methane into the permeate stream.
U.S. Pat. No. 5,352,272, to Dow Chemical, concerns operation of glassy membranes at subambient temperatures to improve selectivity for one gas over another. To achieve an acceptable selectivity for nitrogen over methane using known membrane materials would need an increase over the room-temperature selectivities shown in Table 1 of at least five-fold and more probably seven-fold or eight-fold. It is probable that the methane in the stream would liquefy before a low enough temperature to achieve this selectivity could be reached. Also, in glassy membranes, permeability, which is dominated by the diffusion coefficient, declines with decreasing temperature, so permeabilities, already low, would rapidly decline to an unacceptably low value.
Membrane separations are usually driven by a pressure difference between the feed and permeate sides, the feed side being at high pressure with respect to the permeate side. With a methane-selective membrane, if the bulk of the gas stream being treated has to pass to the permeate low-pressure side, then be recompressed, it is to be expected that this would make for an inefficient and hence costly process. Likewise, the membrane area that is needed to perform the separation is in proportion to the amount of gas that must cross the membrane; if most of the gas in the feed has to permeate the membrane, a much larger membrane area will be needed than if only a few percent of the feed gas has to permeate.
Thus, the separation of nitrogen from methane by means of membranes is a very difficult problem. U.S. Pat. No. 5,669,958 teaches such a separation, accomplished by operating a methane-selective membrane at a low temperature. U.S. Pat. No. 5,647,227 teaches the use of a similarly operated methane-selective membrane in conjunction with cryogenic distillation of nitrogen from methane.
Of course, most raw natural gas that is out of specification in terms of excess nitrogen content will also be out of specification with regard to at least one other component, such as water vapor, acid gas (carbon dioxide and/or hydrogen sulfide) and C.sub.3+ hydrocarbons. As a result, treatment trains are required that provide appropriate, consistent and integrated treatments for multiple contaminants.